Examination of 95 Dall’s porpoise specimens incidentally caught in gillnets, and four collected as beach strandings, indicate that there is significant sexual dimorphism and developmental variation in several aspects of external morphology and coloration. The dorsal fins of males become extremely canted in adulthood, and mature males can be distinguished by this feature alone. Size of the post-anal hump of connective tissue, and caudal peduncle depth also become exaggerated in adult males. The uro-genital color pattern is highly variable, and frosting variations on the dorsal fin and flukes can be used to discern the general age of the individual. The strong sexual dimorphism and small testes of Dall’s porpoise indicate a polygynous mating system. It is suggested that Dall’s porpoise secondary sexual characteristics are used in male-male competition or female choice.